1. Field of the Invention:
The invention relates to an four-order sigma-delta modulator, which implements a feedback and a feedforward configuration, so that the desired noise transfer function is carried out by adjusting the coefficients of the feedback and the feedforward.
2. Description of the Related Art:
With proper design, the sigma-delta modulator can have the advantages of high resolution, simple circuit, no additional trimming circuit and high tolerance for device replacement. As is well-known in sigma-delta modulators, SNR is dependent on the oversampling ratio, the sigma-delta modulator order and quantizer resolution. Theoretically, doubling the oversampling ratio should result in a 3 dB improvement in SNR, and increasing the order of the sigma-delta modulator by one should produce another 6 dB improvement in SNR within the desired bandwidth. A 1-bit (two value) increase in quantizer resolution should yield another 6 dB. Therefore, such a sigma-delta modulator is widely applied in situations requiring higher SNR. As well, digital communication has been growing rapidly during the past few years. For development of digital communication, a three order or more sigma-delta modulator has become a popular design area.
As shown in FIG. 1, a typical high-order sigma-delta modulator feeds back the digital signal Y from an A/D quantizer 108 to the input terminals respectively to the integrators 102, 104 and 106 through a D/A converter 110, so as to form a desired three-order or more noise transfer function. Such a structure can have different noise transfer functions by adjusting the integrator""s output coefficients. However, the feedback configuration in the sigma-delta modulator will cause overload and introduce a complicated circuit when implementing the desired noise transfer function, even though it can have higher resolution and higher SNR with higher orders.
Accordingly, an object of the invention is to provide a four-order sigma-delta modulator, which implements a feedback and a feedforward configuration, so that the desired noise transfer function is carried out by adjusting the coefficients of the feedback and the feedforward.
To realize the above object, the invention provides a four-order sigma-delta modulator comprising: an analog-to-digital converter, to output a digital modulation signal; a digital-to-analog converter, to convert the digital modulation signal into an analog signal and generate a feedback signal; a first integrating network, to receive an external input signal and the feedback signal and generate a first output signal; a second integrating network, connected in series with the first integrating network, to receive the first output signal and a feedback gain signal with a first feedback gain factor, generated by gaining the feedback signal by the first feedback gain factor, and generate a second output signal; a third integrating network, connected in series with the second integrating network, to receive the second output signal and generate a third output signal; a fourth integrating network, connected in series with the third integrating network, to receive the third output signal and a feedback gain signal with a second feedback gain factor, generated by gaining the feedback signal by the second feedback gain factor, and generate a fourth output signal; and an adder, combining a feedforward output signal with a feedforward gain factor, generated by gaining the second output signal by the feedforward gain factor, and a modulation output signal with a modulation gain factor, generated by gaining the fourth output signal by the modulation gain factor, to generate a desired noise transfer function, and outputting a noise transfer signal to the digital-to-analog converter.